1. Field of Invention
This invention advances previous flexural pivot designs by simplifying the manufacture, assembly, and integration into a device.
2. Description of Prior Art
A typical pivot consists of a combination of bearings which provide both radial and axial stiffness, while allowing a low resistance to rotation about an axis. Radial and thrust journal bearings can accomplish this with simple components, but the continual or intermittent sliding contact generates significant resistance to rotation, and material wear limits the service life. Radial and thrust roller bearings can provide the axial and radial stiffness under heavy loads, but require precision roller elements, raceway structures, and lubrication methods. A pre-loaded set of two ball bearings can provide axial and radial stiffness for light loads, but again require precision balls, raceway structures, and lubrication.
Journal, ball, and roller bearings allow a simple co-axial configuration of the fixed and rotational portions. A shaft is attached to the inner diameter of the bearings and the outer diameter of the bearings are attached to a sleeve or housing. The shaft can rotate or the sleeve can rotate. With a rotating sleeve, the shaft can be fixed at one end (cantilevered) or both ends (doubly-supported). Fixing the shaft at both ends (doubly-supported) offers a significantly more rigid structure over a cantilevered attachment, and is required in many applications. The co-axial placement of the shaft and sleeve allow a straight-forward centering of the rotational mass between the axial spacing of the bearings. The centering of the rotational mass between the radial bearing elements is usually desired to uniformly distribute radial loads.
However, journal, ball, and roller bearings have several disadvantages. They require oil or grease lubrication and the associated oil replenishment or grease seals for ensuring a long service life in harsh environments. Lubricants can migrate, decompose, or give off gasses over their working life, and degrade the performance of the bearing. Migration or off-gassing of lubricants may also contaminate surrounding parts of the device in which the bearing is mounted. These contaminants can cause failure of sensitive components such as those found on satellites or inside magnetic data storage drives. In satellites, the vacuum of space will draw-out chemical compounds from the lubricants, which can foul sensors. In magnetic data storage drives, off-gassed chemical compounds can foul critical air bearing surfaces which are designed to operate at air gaps of less than a millionth of an inch. Journal, ball, and roller bearings are also sensitive to contamination; if particles are introduced between the moving surfaces, an increased force is required to roll over the particle. Hard particles will damage the bearing surfaces and hasten bearing wear. Given the sliding wear or rolling resistance, these bearings exhibit a hysteresis-effect; frictional forces oppose motion in both rotational directions. Additionally, the properties of lubricants vary with temperature; at low temperatures an increased force is required to displace lubricants. Further, lubricants are typically non-conductive, which electrically isolates the rotational portion from the fixed portion, allowing a generally un-desirable voltage potential to develop between the two portions.
Advantages of Flexural Pivots:
For applications requiring rotational motion within a limited angular range, +/−30 degrees for example, Lucas produces and markets a line of flexural pivots as described in U.S. Pat. No. 3,811,665. FIG. 1 depicts a Lucas flexural pivot 10. A pair of flexures 13 are attached to the inner diameter of a stationary member 11. The flexures are also attached to the inner diameter of a rotational member 12.
Flexural pivots such as the Lucas pivot provide many advantages over journal bearings, ball, or roller bearings. They require neither lubrication nor the associated seals and oil replenishment systems. Without the temperature-sensitive greases or oils, their performance varies little over a wide range of temperatures. They are not sensitive to contaminants. If fabricated out of metal components, the Lucas flexural pivot can provide a continuous, low-resistance electrical path to eliminate voltage potentials between the moving and stationary portions of the pivot. With no rolling or sliding interactions, the life of a flexural pivot can be many times that of a journal, ball, or roller bearing pivot. With no friction forces to oppose rotational motion regardless of rotational direction, there is no hysteresis-effect exhibited by a flexural pivot.
Further, the flexural members of a flexural pivot provide a restoring force to the pivot, such that it will return to a repeatable angular position when external forces are removed. This is beneficial during assembly to control the relative positioning of the stationary and rotate-able portions. It is also beneficial to the application, when it is desired for the rotate-able portion to return to a pre-determined rotational location.
While the Lucas flexural pivot solved many problems, the design limited its applications. The rotating and fixed housings are both tubular forms, which allow for limited attachment methods. Attachment is necessarily made on the outer diameter of both the fixed and rotating housings, which are the same size and closely spaced along the axis of the pivot. Hence, the mating parts must be closely placed along the axis of the pivot, but not at the same axial location. The simplest of the Lucas pivots offers two separate tubular forms, one fixed and one rotational. The fixed portion is held at one end and the rotational portion is at the other end of the pivot axis. The fixed portion cannot be held at both ends of the pivot's axis. Hence, the rotational portion must be cantilevered. This cantilevered attachment offers significantly lower stiffness than a doubly-attached shaft, as journal, ball, and roller bearing pivots allow.
Further, the rotating and fixed housings of the Lucas pivot are complex forms, generally requiring wire electron-discharge machining, are well as conventional lathe and milling machine operations. Assembly of the flexures into the inner diameters of these tubular housings is difficult, especially for small pivots and small tubular diameters.
Objects and Advantages:
The current invention simplifies the fabrication of flexural pivot components, uncomplicates the assembly, improves upon methods of attachment, and allows scaling to smaller sizes. These features allow integration of this flexural pivot into a wider range of applications. These applications can now take advantage of the benefits of flexural pivots in applications where journal, ball, or roller bearing pivots have been the only choice.